Transcript Slide 1

BASIC ELECTRONICS
Bipolar Junction Transistor
Department of Electronics and Communication
School of Engineering, Manipal University Jaipur
Syllabus
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Introduction to Bipolar Junction Transistor
BJT Operation
BJT Configurations
Tutorials
BJT Biasing
Tutorials
BJT Amplifier
Tutorials
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Reference Books
1. “Electronic Devices and Circuit Theory” by
Boylestad & Nashelsky,
2. “Integrated Electronics” by Millman & Halkias,
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Introduction
• Solid state transistor was invented by a team of scientists at
Bell laboratories during 1947-48
• It brought an end to vacuum tube era
• Advantages of solid state transistor over vacuum devices:
– Smaller size, light weight
– No heating elements required
– Lower power consumption and operating voltages
– Low price
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Introduction
Figure showing relative sizes of
transistor, IC and LED
Figure showing different transistor packages
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Introduction
• Bipolar Junction Transistor (BJT) is a sandwich consisting of
three layers of two different types of semiconductor
• Two kinds of BJT sandwiches are: NPN and PNP
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Introduction
• The three layers of BJT are called Emitter, Base and Collector
• Base is very thin compared to the other two layers
• Base is lightly doped. Emitter is heavily doped. Collector is
moderately doped
• NPN – Emitter and Collector are made of N-type
semiconductors; Base is P-type
• PNP – Emitter and Collector are P-type, Base is N-type
• Both types (NPN and PNP) are extensively used, either
separately or in the same circuit
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Introduction
• Transistor symbols:
Note: Arrow direction from P to N (like diode)
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Introduction
• BJT has two junctions – Emitter-Base (EB) Junction and
Collector-Base (CB) Junction
• Analogous to two diodes connected back-to-back:
– EB diode and CB diode
• The device is called “bipolar junction transistor” because
current is due to motion of two types of charge carriers – free
electrons & holes
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Transistor Operation
• Operation of NPN transistor is discussed here; operation of
PNP is similar with roles of free electrons and holes
interchanged
• For normal operation (amplifier application)
– EB junction should be forward biased
– CB junction should be reverse biased
• Depletion width at EB junction is narrow (forward biased)
• Depletion width at CB junction is wide (reverse biased)
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Transistor Operation
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Transistor Operation
Un-biased transistor showing barriers at the junctions
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Transistor Operation
C-B junction is reverse biased – increased barrier height
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Transistor Operation
E-B junction is forward biased – aids charge flow
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Transistor Operation
Electron-hole combination – leads to small base current
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Transistor Operation
• When EB junction is forward biased, free electrons from
emitter region drift towards base region
• Some free electrons combine with holes in the base to form
small base current
• Inside the base region (p-type), free electrons are minority
carriers. So most of the free electrons are swept away into the
collector region due to reverse biased CB junction
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Transistor Operation
•
1.
Three currents can be identified in BJT
Emitter current
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2.
This is due to flow of free electrons from emitter to base
Results in current from base to emitter
Base current
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3.
This is due to combination of free electrons and holes in the base
region
Small in magnitude (usually in micro amperes)
Collector current
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Has two current components:
One is due to injected free electrons flowing from base to collector
Another is due to thermally generated minority carriers
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Transistor Operation
C
IC
C
IB
IB
B
B
IE
E
IC
IE
E
NPN
PNP
• Note the current directions in NPN and PNP transistors
• For both varieties:
---(1)
I E  IC  I B
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Transistor Operation
• As noted earlier, collector current has two components:
– One due to injected charge carriers from emitter
– Another due to thermally generated minority carriers
• Both results in current in the same direction. Hence
--- (2)
I  I  I
C
dc E
CBO
αdc is the fraction of charge carriers emitted from emitter, that
enter into the collector region
ICBO is the reverse saturation current in CB diode
I I
--- (3)
 dc  C CBO
IE
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Transistor Operation
• As approximation, we can neglect ICBO compared to IE and IC
• Hence approximate equations are: I   I
C
 dc 
dc E
IC
IE
• Like the reverse saturation current of ordinary diode, ICBO also
doubles for every 10o C rise in temperature.
• So ICBO cannot be neglected at higher temperatures
• The parameter αdc is called common-base dc current gain
• Value of αdc is around 0.99
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Transistor Operation
I  I  I
dc E
CBO
• We have C
• Substituting for IE, we get I C   dc I C  I B   I CBO
(1   dc ) I C   dc I B  I CBO
 dc
I CBO
IC 
IB 
(1   dc )
(1   dc )
I C  dc I B  I CEO --- (4)
• Where
 dc
 dc 
(1   dc )
and
I CEO
I CBO

  dc  1I CBO
(1   dc )
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Transistor Operation
• Equations (2) and (4) are two alternate forms of BJT current
equation
• Since value of αdc is around 0.99, ICEO >> ICBO
• However, ICEO is still very small compared to IC
IC
• Hence approximation of (4) gives: I C   dc I B or  dc 
IB
• Parameter βdc is called common emitter dc current gain
• Values of αdc and βdc vary from transistor to transistor. Both
αdc and βdc are sensitive to temperature changes
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Problems
1.
A BJT has alpha (dc) 0.998 and collector-to-base reverse sat
current 1μA. If emitter current is 5mA, calculate the
collector and base currents.
(Ans: 4.99 mA, 10 μA)
2. An npn transistor has collector current 4mA and base current
10 μA. Calculate the alpha and beta values of the transistor,
neglecting the reverse sat current ICBO
(Ans: 0.9975, 400)
3. In a transistor, 99% of the carriers injected into the base cross
over to the collector region. If collector current is 4mA and
collector leakage current is 6 μA, Calculate emitter and base
currents
(Ans: 4.034 mA, 34 μA)
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Transistor Configurations
• BJT has three terminals
• For two-port applications, one of the BJT terminals needs to
be made common between input and output
Input
2-port
device
Output
• Accordingly three configurations exist:
– Common Base (CB) configuration
– Common Emitter (CE) configuration
– Common Collector (CC) configuration
• (The last one is not discussed in this course)
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Transistor Configurations
• Common Base configuration
(Resistors are not shown here
for simplicity)
• Base is common between input and output
– Input voltage: VEB
Input current: IE
– Output voltage: VCB Output current: IC
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Transistor Configurations
• CB Input characteristics
– A plot of IE versus VEB
for various values of VCB
– It is similar to forward
biased diode
characteristics
– As VCB is increased, IE
increases only slightly
– Note that second letter in
the suffix is B (for base)
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Transistor Configurations
• Input resistance ri
VEB
ri 
with VCB const
I E
• Voltage amplification factor AV
VCB
AV 
with I E const
VEB
• Both can be determined from the CB input characteristics
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Transistor Configurations
VEB1  VEB 2
ri 
I E1  I E 2
VCB1  VCB 2
AV 
VEB1  VEB 2
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Transistor Configurations
• CB Output characteristics
– A plot of IC versus VCB for various values of IE
– Three regions are identified: Active, Cutoff, Saturation
– Active region:
• E-B junction forward biased
• C-B junction reverse biased
• IC is positive, VCB is positive
• IC increases with IE
• For given IE, IC is almost constant; increases only
slightly with increase in VCB. This is due to base-width
modulation (Early effect)
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Transistor Configurations
CB Output characteristics
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Transistor Configurations
• Base Width modulation
– As the reverse bias voltage VCB is increased, the depletion
region width at the C-B junction increases. Part of this
depletion region lies in the base layer. So, effective base
width decreases. Hence number of electron-hole
combination at the base decreases. So base current reduces
and collector current increases.
– Note that IE = IC + IB
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Transistor Configurations
• When IE = 0, IC = ICBO
– ICBO is collector to base current with emitter open
– Below this line we have cut-off region
– Here both junctions are reverse biased
• Region to the left of y-axis (VCB negative) is saturation region
– Here both junctions are forward biased
– IC decreases exponentially, and eventually changes
direction
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Transistor Configurations
• Output resistance ro
VCB
rO 
with I E const
I C
• Current amplification factor AI or αac
I C
 ac 
with VCB const
I E
• Both can be measured from output characteristics
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Transistor Configurations
• Common Emitter configuration
(Resistors are omitted for simplicity)
• Emitter is common between input and output
– Input voltage: VBE
– Output voltage: VCE
Input current: IB
Output current: IC
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Transistor Configurations
CE input characteristics
• Plot of IB versus VBE for
various values of VCE
• Similar to diode
characteristics
• As VCE is increased, IB
decreases only slightly
• This is due to base-width
modulation
• Note that second suffix is E
(for emitter)
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Transistor Configurations
• CE output characteristics
– A plot of IC versus VCE for various values of IB
– Three regions identified: Active, Cut-off, Saturation
– Active region:
• Linear region in the output characteristics
• E-B junction forward biased
• C-B junction reverse biased
• IC increases with IB
• For given IB, IC increases slightly with increase in VCE;
this is due to base-width modulation (Early effect)
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Transistor Configurations
CE output characteristics
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Transistor Configurations
• Note that VCE = VCB + VBE
• So if VCE is increased, effectively VCB also increases
• For saturation to take place, C-B junction should be forward
biased.
• This happens when VCE is approximately 0.3 V (or less) for Si
• Note that when VCE= 0.3V, and VBE= 0.6 V, VCB= –0.3V (a
forward bias of 0.3 V)
• So region to the left of the vertical line VCE=VCE(sat)=0.3V (for
silicon) is considered as saturation region
• Region below IB=0 line (or IC=ICEO) is cut-off region
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Transistor Configurations
• ICEO is much larger than ICBO because of the relation:
I CEO 
I CBO
1   dc
• Note that value of αdc is around 0.99
• The values of αdc & αac, and βdc & βac are almost the same.
Hence the subscripts can be omitted for simplicity
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Transistor Configurations
• Input resistance ri
VBE
ri 
with VCE  const
I B
• Voltage gain AV
V
AV  CE with I B  const
VBE
• Output resistance ro
VCE
ro 
with I B  const
I C
• Current gain AI or βac
I C
 ac 
with VCE  const
I B
• All these parameters can be determined from CE characteristics
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Transistor Configurations
• Experimental
setup for
determining CE
characteristics:
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Tutorials
A Ge transistor with β = 100 has collector-to-base leakage
current of 5 µA. If the transistor is connected in commonemitter operation, find the collector current for base current
(a) 0 (b) 40 µA.
Sol: Given that ICBO = 5µA, and β = 100
We know that
1.
When IB = 0,
IC = ICEO = (β+1)ICBO = 505 µA
When IB = 40 µA,
IC = βIB + ICEO
= (100 × 40 × 10–6) + (505 × 10–6)
= 4.505 mA
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Tutorials
2. A Ge Transistor has collector current of 51 mA when the base
current is 0.4 mA. If β = 125, then what is its collector cutoff
current ICEO?
(Ans: 1 mA)
3. In a transistor circuit, when the base current is increased from
0.32 mA to 0.48 mA, the emitter current increases from 15
mA to 20 mA. Find αac and βac values.
(Ans: 0.968, 30.25)
4. A transistor with α = 0.98 and ICBO = 5 µA has IB = 100 µA.
Find IC and IE.
(Ans: 5.15 mA, 5.25 mA)
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Transistor Biasing
• One of the most common applications of transistor is in
amplifiers.
• E-B junction should be forward biased; C-B junction
should be reverse biased (active region)
• For faithful amplification we require that transistor be
operated in active region throughout the duration of input
signal.
• To ensure this, proper dc voltages should be applied.
This is called Biasing.
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Operating point
When no input signal is applied to transistor circuit, and
only dc voltages are supplied, currents IC, IB and voltage
VCE will have certain values.
If these values are plotted over the transistor output
characteristics, the point we get is called ‘Operating point’.
It is also called ‘Quiescent point’ or just Q-point.
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• In above figure, currents IBQ, ICQ and voltage VCEQ are
plotted as point Q. In practice, we have to choose Q-point
according to our requirement. If we want to operate in the
middle of active region, we may choose Q as Q-point.
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•
If we want to operate near saturation, we may choose
Q’ as Q-point.
• If we want to operate near cutoff, we may choose Q’’ as
Q-point.
• Note that if no biasing is used, Q-point will be in the
origin of graph.
• So, biasing is used to fix the Q-point according to our
need.
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Type of Biasing in Transistor
Fixed bias
Voltage divider bias
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Fixed Bias
• Base resistor RB is connected to Vcc (Instead of VBB).
Negative terminal of Vcc is not shown. It is assumed to
be at ground.
• Applying KVL to the input,
V 
V
IB  CC BE
R
B
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Vcc is constant, VBE is almost constant (0.7V for silicon). So
by selecting proper RB, we can fix IB as required.
Applying KVL to output side we get:
• IC is related to IB by β
• So, VCE can be fixed by selecting proper RC.
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Load Line
We have,
This is an equation of straight line with VCE and IC as two
variables. This straight line is called load line. Now, output
characteristic is also a function of same two variables.
If RB and RC are held constant and VCC is varied, then load
line shifts, maintaining same slope.
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• If RB and RC are held constant and VCC is varied, then
load line shifts, maintaining same slope. From these
graphs we infer that:
• with everything else held constant, if RB is increased,
transistor goes towards cutoff.
• if RB is decreased, transistor goes towards saturation.
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• With everything else held constant, if RC is increased,
transistor goes towards saturation.
• if RC is decreased, transistor goes towards active
region.
• With everything else held constant, if VCC is increased,
transistor goes towards active region.
• if VCC is decreased, transistor goes towards saturation.
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Advantages of fixed bias
• Simple to analyze and design
• Uses very few circuit components
Disadvantages of fixed bias
• Q-point is not stable. i.e., if temperature varies, β will
vary, hence IC will vary.
• If transistor is replaced by another transistor having
different β then Q-point will shift.
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Voltage divider bias or Self bias
• Uses two resistors R1 and R2 instead of RB. RE is
connected between emitter and ground.
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• Input side of the above circuit is redrawn below,
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• VTH is the open circuit voltage between points A & B in
fig (a) given by,
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• RTH is the resistance between A & B with VCC replaced
by short circuit.
• Applying KVL to the input loop,
Substituting
and rearranging, we get
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• Applying KVL to the output loop, we get
Rearranging, we get
Also,
Where, VC is voltage from collector to ground.
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• Also,
Where, VE is voltage from emitter to ground.
Since β>> 1, we have (β+1) ≈ β. If βRE >> RTH, then
equation of IB reduces to,
Now,
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• Since equation for IC does not contain β, we say that IC is
independent of temperature variation and transistor
replacement.
Advantages of voltage divider bias
. Q-point is stable against variation in temperature and
replacement of transistor.
Disadvantages of voltage divider bias
• Analysis and design are complex
• More circuit components required
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Problem
1. Draw the DC load line and mark the Q- point on fixed bias cct.
Assume Beta DC=100 and neglect Base –Emitter voltage.
(Vcc=30V,RB=1.5 Mohm, RC=5 Kohm).
Ans: VCE,max=30v,VCEQ=20v,ICQ=2mA
2. In a fixed bias cct. Find the base current required to establish
VCE=6v, also find RB & IE, (VBE=0.7v, Beta DC=120,
VCC=12v,RC=2.2.Kohm).
Ans: IB=22.75 uA, RB=497 Kohm
3. Determine the region in which the transistor operates.
(VBE=0.2v,RB=120kohm,RC=1kohm,VCC=15v,Beta DC=120).
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End of Module 3
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